In many semiconductor applications, it is necessary or at least desirable to form conductive bumps on the input and output terminals (bond pads) of a semiconductor die. The most common applications where conductive bumps are used include tape automated bonding (TAB), flip-chip attachment of a die to an intermediate substrate, and direct chip attachment (DCA) of a die to a user substrate. A variety of methods exist for forming conductive bumps for these applications. One method involves depositing metal onto the bond pads, for example by electro-plated deposition or through evaporative deposition. Due to high manufacturing costs, however, use of these deposition techniques is not always favored.
As a replacement for methods which form conductive bumps through deposition, some semiconductor manufacturers have resorted to a method known as ball bumping. Ball bumping utilizes a standard wire bonding tool. Unlike conventional wire bonding, where a bond is first formed to the bond pad of a die and then a second bond is formed to a lead or a conductive pad on a substrate (with the wire being continuous between the two bonds), conventional ball bonding utilizes only one bond. A capillary of the wire bonding tool brings a conductive wire toward the bond pad on which a bump is to be formed. The conductive wire is heated to melt the metal at an end of the wire to form a ball, and the capillary forcibly presses the ball against the bond pad. The wire is then cut, leaving a kiss-shaped bump (known as a ball bump) on the bond pad. Because the ball bump is not flat, a subsequent flattening operation may be necessary for some applications. For example, in TAB bonding, it is important that the ball bump be flat so that fragile TAB leads can be bonded to the ball bumps reliably and repeatedly, without slipping off the bump surface.
A few methods for flattening the ball bumps have been proposed. One method involves forming ball bumps on all bond pads of the semiconductor die, and subsequently coining all ball bumps at the same time by pressing the die against a flat surface. A problem with this technique is that it involves two separate operations and handling steps, one to form the bumps, and one to coin them. Another proposed method is to utilize the capillary of the wire bonding tool to shear off the protruding tail portion of the ball bump. A disadvantage of a shearing technique is that the wire can break inside the capillary, thereby clogging it. Others have proposed individually coining each ball bump as it is formed. For example, after forming the ball bump, a flat metal wedge may be used to squash each bump after being formed. It has also been proposed that the capillary itself may be used to flatten the ball bump; however, in processes which propose use of the capillary for flattening, a protrusion on the bump surface still remains.
Accordingly, there is a need for an improved ball bumping process wherein the ball bump formed has an essentially flat top surface, and wherein the process for forming such bumps can be done with minimal manufacturing time and expense.